1. Field of the Invention
The invention relates in general to a light-emitting device and manufacturing method thereof, and more particularly to a light-emitting device with open-loop control and manufacturing method thereof.
2. Description of the Related Art
White light is a mixed light of several different color lights. Any white light visible to the naked eyes is a mixed light composed of at least two color lights with different wavelengths. For example, when one's naked eyes receive a red light, a green light and a blue light at the same time or by the blue light and a yellow light at the same time, the received light is perceived as a white light. There are three common light sources, namely, the fluorescent lamp with color temperature approximates to 7500K, the filament lamp with color temperature approximates to 3000K, and the white light emitting diode (LED) that is still under development.
There are five methods to manufacture the white light LED. According to the first method, three different currents respectively provided to three LEDs made from indium gallium aluminum phosphide (InGaAlP), gallium phosphide (GaP) and gallium nitride (GaN) are controlled to drive the three LEDs to emit a red light, a green light and a blue light, and then a white light is generated by mixing the three lights. According to the second method, two LEDs made from gallium nitride (GaN) and indium gallium aluminum phosphide (InGaAlP) respectively receive different currents that are controlled to drive the LEDs to emit a blue light and a yellowish green light or a green light and a red light, and further a white light is generated by mixing the two colored lights. According to the third method provided by the Nichia Chemical Ltd of Japan in 1996, a blue-light LED made from indium gallium nitride (InGaN) is combined with a yttrium aluminum garnet fluorescent powder that emits a yellow light, then the two color lights are mixed to generate a white light. The third method is disclosed in Taiwanese Patent Publication No. 1561771 and U.S. Pat. No. 5,998,925. According to the fourth method that provided by the Sumitomo Electric Industries Ltd of Japan in the January of 1999, a white light LED made from zinc selenide (ZnSe) is used. At first, a cadmium zinc selenide (CdZnSe) thin film is formed on a zinc selenide single chip substrate. The thin film emits a blue light when the thin film is provided with a current, meanwhile portion of the blue light projects to the substrate and emits a yellow light. Then the blue light and the yellow light that are complementary to each other are mixed to form a white light. According to the fifth method, a UV white light LED is used. A UV light is used to excite different fluorescent powders to generate different fluorescent lights, and then the fluorescent lights are further mixed to form a white light.
Above the five methods that are implemented to manufacture white light LEDs, the first method and the second method are able to automatically control the white light chromaticity coordinate by changing the currents to compensate the spectrum of the mixed light. The white light chromaticity coordinates generated according to the remaining three methods using fluorescent materials are easily affected by the color light emitted by the LEDs or the fluorescent materials, and are unable to automatically control the white light at a fixed chromaticity coordinate to compensate the spectrum of the mixed light. Despite the first method is able to adjust the currents of the three chips to compensate the spectrum of the mixed light and automatically control the white light chromaticity coordinate, the currents of the three chips need to be controlled respectively, making the control circuit even more complicated and costive. Despite the second method is also able to adjust the currents of the two chips to compensate the spectrum of the mixed light and automatically control the white light chromaticity coordinate, the problems that occur in the first method also occur in the second method.